We study the plasmon excitations and the electromagnetic response of the magnetically doped single-layer and multilayer “helical metals,” which emerge at the surfaces of topological insulators. For the single-layer case, we find a “spin-plasmon” mode with the rotating spin texture due to the combination of the spin-momentum locking of helical metal and the Hall response from magnetization. For the multilayer case, we investigate the electromagnetic response due to the plasmon excitations, including the Faraday rotation for the light propagating normal to the helical metal layers and an additional optical mode with the frequency within the conventional plasmon gap for the light propagating along the helical metal layers.

Magnetic fluctuations near to quantum criticality can have profound effects. They lead to characteristic scaling at high temperature which may ultimately give way to a reconstruction of the phase diagram and the formation of new phases at low temperatures. The ferromagnet UGe${}_2$ is unstable to $p$-wave superconducting order—an effect presaged by the superfluidity in ${}^3$He—whereas in CeFePO fluctuations drive the formation of spiral magnetic order. Here we develop a general quantum order-by-disorder description of these systems that encompasses both of these instabilities within a unified framework. This allows us to demonstrate that in fact these instabilities intertwine to form a pair density wave.

We study a generalized Kane-Mele-Hubbard model with third-neighbor hopping, an interacting two-dimensional model with a topological phase transition as a function of third-neighbor hopping, by means of the determinant projector quantum Monte Carlo method. This technique is essentially numerically exact on models without a fermion sign problem, such as the one we consider. We determine the interaction dependence of the $Z_2$ topological insulator/trivial insulator phase boundary by calculating the $Z_2$ invariants directly from the single-particle Green's function. The interactions push the phase boundary to larger values of third-neighbor hopping, thus, stabilizing the topological phase. The observation of boundary shifting entirely stems from quantum fluctuations. We also identify qualitative features of the single-particle Green's function which are computationally useful in numerical searches for topological phase transitions without the need to compute the full topological invariant.

We show through both theoretical arguments and numerical calculations that graphene discerns an unconventional sequence of quantized Hall conductivity, when subject to both magnetic fields ($B$) and strain. The latter produces time-reversal symmetric pseudo/axial magnetic fields ($b$). The single-electron spectrum is composed of two interpenetrating sets of Landau levels (LLs), located at $\pm \sqrt{2n|b\pm B|}$, $n=0,1,2,...$. For $b>B$, these two sets of LLs have opposite chiralities, resulting in oscillating Hall conductivity between 0 and $\mp 2e^2/h$ in electron and hole doped systems, respectively, when the chemical potential is tuned in the vicinity of the neutrality point. The electron-electron interactions stabilize various correlated ground states, e.g., spin-polarized, quantum spin Hall insulators at and near the neutrality point, and possibly the anomalous Hall insulating phase at incommensurate filling $\sim$$B$. Such broken-symmetry ground states have similarities as well as significant differences from their counterparts in the absence of strain. For realistic strength of magnetic fields and interactions, we present scaling of the interaction-induced gap for various Hall states within the zeroth Landau level.

Nonreciprocity of complex metamolecules based on the collapse of parity-time symmetry is investigated through an application to an optical waveguide array, demonstrating the strong dependency on the phase of eigenspectra. Contrary to the intuitive expectation, we reveal that more rapid and tunable nonreciprocal dynamics is achieved in the regime where the real and complex phases coexist and, furthermore, with the benefit of significantly reduced gain. It is shown that this paradoxical behavior originates from the effect of “spatial trapping modes,” which hinder the energy transfer within the metamolecule due to decreased effective coupling between waveguides. As an application, we demonstrate the novel phenomenon of “reversible nonreciprocity,” based on the manipulation of a single trapping mode.

We develop a classification of composite operators without gradients at Anderson-transition critical points in disordered systems. These operators represent correlation functions of the local density of states (or of wave-function amplitudes). Our classification is motivated by the Iwasawa decomposition for the field of the pertinent supersymmetric $\sigma$ model: The scaling operators are represented by “plane waves” in terms of the corresponding radial coordinates. We also present an alternative construction of scaling operators by using the notion of highest-weight vector. We further argue that a certain Weyl-group invariance associated with the $\sigma$-model manifold leads to numerous exact symmetry relations between the scaling dimensions of the composite operators. These symmetry relations generalize those derived earlier for the multifractal spectrum of the leading operators.

Ripples in graphene can occur due to strain or thermal fluctuations and stabilize the two-dimensional material. Molecular dynamics simulations show that thermally induced ripples in graphene lead to angular deviations of the surface normal that agree with previous electron diffraction experiments [J. C. Meyer et al., Nature (London) 446, 60 (2007)]. We discover scaling relationships for the average angular deviations as a function of size of the graphene sheet $L$ and averaging radius $R$. The average angle scales as $\exp \{-c{(R/L)}^{\alpha }\}$ with a scaling exponent $\alpha =1$ for single layer, $5/4$ for bilayer, and $5/3$ for trilayer graphene, respectively. We show how these angular deviations depend on temperature, strain, and layer numbers. The scaling relations can provide guidance to the optimization of properties that are sensitive to out-of-plane distortions.

We present an ab initio theoretical analysis of the temperature-dependent stability of inherently nanolaminated (Cr${}_{1-x}$Mn${}_x$)${}_2$AlC. The results indicate energetic stability over the composition range $x$ $=$ 0.0 to 0.5 for temperatures ≥600 K. Corresponding thin film compounds were grown by magnetron sputtering from four elemental targets. X-ray diffraction in combination with analytical transmission electron microscopy, including electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy analysis, revealed that the films were epitaxial (0001)-oriented single-crystals with $x$ up to 0.16.

The semimetallic group-V elements display a wealth of correlated electron phenomena due to a small indirect band overlap that leads to relatively small, but equal, numbers of holes and electrons at the Fermi energy with high mobility. Their electronic bonding characteristics produce a unique crystal structure, the rhombohedral A7 structure, which accommodates lone pairs on each site. Here, we show via single-crystal and synchrotron x-ray diffraction that antimony arsenide (SbAs) is a compound and the A7 structure can display chemical ordering of Sb and As, which were previously thought to mix randomly. Formation of this compound arises due to differences in electronegativity that are common to IV-VI compounds of average group V such as GeTe, SnS, PbS, and PbTe, and also ordered intraperiod compounds such as CuAu and NiPt. High-temperature diffraction studies reveal an order-disorder transition around 550 K in SbAs, which is in stark contrast to IV-VI compounds GeTe and SnTe that become cubic at elevated temperatures but do not disorder. Transport and infrared reflectivity measurements, along with first-principles calculations, confirm that SbAs is a semimetal, albeit with a direct band separation larger than that of Sb or As. Because even subtle substitutions in the semimetals, notably Bi${}_{1-x}$Sb${}_x$, can open semiconducting energy gaps, a further investigation of the interplay between chemical ordering and electronic structure on the A7 lattice is warranted.

We investigated the spin reorientation transition during the growth of Fe(110) films on W(110) at 250 °C using the in situ nuclear resonant scattering of x rays. The measurements as a function of the grazing incidence angle demonstrated that the spin reorientation proceeds via an intermediate, noncollinear magnetic state, and with increasing thickness, the magnetization reorientation from the [1$\overline{1}$0] to the [001] direction is activated at the film surface and completed at the Fe/W interface. Furthermore, as the temperature was decreased from 250 °C to 160 °C, the temperature-driven spin reorientation transition (SRT) between the [001] and [1$\overline{1}$0] directions was observed with a similar noncollinear transient magnetic state. The thickness-temperature SRT phase diagram was schematically drawn.

Topological behavior can be masked when disorder is present. A topological insulator, either intrinsic or interaction induced, may turn gapless when sufficiently disordered. Nevertheless, the metallic phase that emerges once a topological gap closes retains several topological characteristics. By considering the self-consistent disorder-averaged Green function of a topological insulator, we derive the condition for gaplessness. We show that the edge states survive in the gapless phase as edge resonances and that, similar to a doped topological insulator, the disordered topological metal also has a finite, but nonquantized topological index. We then consider the disordered Mott topological insulator. We show that within mean-field theory, the disordered Mott topological insulator admits a phase where the symmetry-breaking order parameter remains nonzero but the gap is closed, in complete analogy to “gapless superconductivity” due to magnetic disorder.

The penetration of magnetic flux fronts in the optimally doped iron-based superconductor Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($x=0.07\pm 0.005$) is studied by means of magneto-optical imaging and Bitter decoration. The higher-order analysis of roughening and growth of the magnetic flux front reveals anomalous scaling properties, indicative of non-Gaussian correlations of the disorder potential. While higher-order spatial correlation functions reveal multifractal behavior for the roughening, the usual Kardar-Parisi-Zhang growth exponent is found. Both exponents are found to be independent of temperature. The scaling behavior is manifestly different from that found for other modes of flux penetration, such as that mediated by avalanches, suggesting that multiscaling is a powerful tool for the characterization of roughened interfaces. We propose a scenario for vortex penetration based on two-dimensional percolation and cluster aggregation for an inhomogeneously disordered superconductor.

We compare tunneling density of states (TDOS) into two ultrathin Ag films, one uniform and one granular, for different degrees of disorder. The uniform film shows a crossover from Altshuler-Aronov (AA) zero bias anomaly to Efros Shklovskii (ES) like Coulomb gap as the disorder is increased. The granular film, on the other hand, exhibits AA behavior even deeply in the insulating regime. We analyze the data and find that granularity introduces a new regime for the TDOS. While the conductivity is dominated by hopping between clusters of grains and is thus insulating, the TDOS probes the properties of an individual cluster which is “metallic.”

The Mn spin correlations were studied near the $O^{\prime }$-$O$ phase transition at $T_{\text{JT}}=750$ K up to 950 K with ${}^{17}\mathrm{O}$ and ${}^{139}\mathrm{La}$ NMR in a stoichiometric ${\mathrm{LaMnO}}_3$ crystalline sample. The measured local hyperfine fields originate from the electron density transferred from the $e_g^{}$ and $t_{2g}^{}$ orbitals to the 2$s$(O) and 6$s$(La) orbits, respectively. By probing the oxygen nuclei, we show that the correlations of the Mn spins are ferromagnetic in the $\mathit{ab}$ plane and robust up to $T_{\text{JT}}$, whereas along the $c$ axis they are antiferromagnetic and start to melt below $T_{\text{JT}}$, at about 550 K. Above $T_{\text{JT}}$, the ferromagnetic Mn-Mn exchange interaction is found isotropic. The room-temperature orbital mixing angle, ${\phi }_{\mathrm{NMR}}=109\pm 1.5{}^{\circ }$, of the $e_g^{}$ ground state is close to the reported value which was deduced from structural data on Jahn-Teller distorted ${\mathrm{MnO}}_6$ octahedra. For $T>T_{\text{JT}}$, ${\mathrm{LaMnO}}_3$ can be described in terms of nonpolarized $e_g$ orbitals since both $e_g$ orbitals are equally occupied.

We demonstrate an on-demand spatial control of excitonic magnetic lattices for the potential applications of excitonic-based quantum optical devices. A two-dimensional magnetic lattice of indirect excitons can form a transition to a one-dimensional lattice configuration under the influence of external magnetic bias fields. The transition is identified by measuring the spatial distribution of two-dimensional photoluminescence for several values of the external magnetic bias fields. The number of trapped excitons is found to increase between sites along a perpendicular direction exhibiting a two- to one-dimensional lattice transition. This work may apply for various controllable quantum simulations, such as superfluid-Mott-insulators, in quantum optical devices.

The quantum capacitance model is applied to obtain an exact solution for the space-resolved carrier density in a multigated doped graphene sheet at zero temperature, with quantum correction arising from the finite electron capacity of the graphene itself taken into account. The exact solution is demonstrated to be equivalent to the self-consistent Poisson-Dirac iteration method by showing an illustrative example, where multiple gates with irregular shapes and a nonuniform dopant concentration are considered. The solution therefore provides a fast and accurate way to compute spatially varying carrier density, on-site electric potential energy, as well as quantum capacitance for bulk graphene, allowing for any kind of gating geometry with any number of gates and any types of intrinsic doping.

We report on the coherent control of terahertz (THz) spin waves in a canted antiferromagnet yttrium orthoferrite, YFeO${}_3$, associated with a quasiferromagnetic (quasi-FM) spin resonance at a frequency of 0.3 THz, using a single-incident THz pulse. The spin resonance is excited impulsively by the magnetic field component of the THz pulse. The intrinsic dielectric anisotropy of YFeO${}_3$ in the THz range allows for coherent control of both the amplitude and the phase of the excited spin wave. The coherent control is based on simultaneous generation of two interfering phase-shifted spin waves whose amplitudes and relative phase, dictated by the dielectric anisotropy of the YFeO${}_3$ crystal, can be controlled by varying the polarization of the incident THz pulse with respect to the crystal axes. The spatially anisotropic decay of the THz-excited FM spin resonance in YFeO${}_3$, leading to an increasingly linear polarization of the THz oscillation at the spin resonance frequency, suggests a key role of magnon–phonon coupling in spin-wave energy dissipation.

We investigate the effect of helical Majorana fermions at the surface of superconducting topological insulators (STIs) on the Josephson current by referring to possible pairing states of Cu-doped Bi${}_2$Se${}_3$. The surface state in the present STI has a spin helicity because the directions of spin and momentum are locked to each other. The Josephson current-phase relation in an STI/$s$-wave superconductor junction shows robust $\sin \left(2\phi \right)$ owing to mirror symmetry, where $\phi$ denotes the macroscopic phase difference between the two superconductors. In contrast, the maximum Josephson current in an STI/STI junction exhibits a nonmonotonic temperature dependence depending on the relative spin helicity of the two surface states. Detecting these features qualifies as distinct experimental evidence for the identification of the helical Majorana fermion in STIs.

We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on ab initio molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The formalism and the numerical aspects of the technique are described in detail. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within ab initio and classical molecular dynamics frameworks. In particular, we examine from first principles the behavior of force constants upon the dynamical stabilization of the body centered phase of Zr, and show that they become more localized. We also calculate the phase diagram for ${}^4$He modeled with the Aziz et al. potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.